The invention relates to a method for charging a capacitive actuator, in particular for a fuel injection valve of an internal combustion engine. The invention also relates to a device for carrying out the method.
One of the advantages when actuating fuel injection valves of an internal combustion engine by piezo actuators instead of solenoids is the short switching time of the actuators, which leads to steep needle edges and low degrees of variation of the injected quantities of fuel. From the point of view of combustion technology, charging times that are as short as possible are to be aimed at.
In order to achieve a more gentle combustion profile, the quantity of fuel is divided into a pre-injection quantity and main injection quantity, which permits slower combustion and thus makes it possible to reduce the combustion noise. The actuators have previously been actuated with a constant charging and discharging time (a duration of the transfer of charge from a power source to the actuator, or vice versa), which must be very short (for example 100 xcexcs) so that a predefined pre-injection fuel quantity can still be injected even in the highest load range or rotational speed range of the internal combustion engine.
The charging process takes place, for example, as a ringing process which includes the charging from one charge source (of a series connection of a charging capacitor and of a recharging capacitor) via a recharging coil to the actuator. An inductance of the recharging coil determining, together with capacitances of the recharging capacitors and of the actuator, the time constant for the charging and discharging processes (the charging and discharging time). Such a device is known from German Patent DE 196 52 801.
German Patent DE 195 29 667 C2 discloses a configuration for the actuation of two piezoelectric actuators in which the frequency of the oscillating circuits in which the piezoelectric actuators are disposed can be changed in order to compensate for temperature effects and aging effects.
Published, Non-Prosecuted German Patent Application DE 197 14 607 A1 describes a method for incrementally charging and discharging a piezoelectric element. The recharging process is switched over to a specific point in time after the start of charging from a charging path with a resistor and a capacitor to a charging path with a coil and a further capacitor. The discharging process takes place in reverse order.
However, the short charging times lead to high noise emissions in frequency ranges which are unpleasant for human ears. This is felt to be very troublesome, for example in a motor vehicle, if the combustion noises are low when the internal combustion engine is idling.
It is accordingly an object of the invention to provide a method and a device for charging a capacitive actuator which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which makes possible a significant reduction in the noise emissions of the actuator.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for charging a capacitive actuator from a charge source through a series circuit formed of a recharging capacitor and a recharging coil, and for discharging the actuator into the recharging capacitor having a much smaller capacitance than the charge source. The method includes the steps of dimensioning the recharging capacitor to have a maximum capacitance for a predefined maximum charging time; and reducing the capacitance of the recharging capacitor to a predefined value at a specific point in time after a start of a charging process for achieving a shorter charging time.
The charging times and the discharging times of the actuator can be varied, in particular in a low-load and idling range of the internal combustion engine, by various measures during the charging process, for example in a range between 100 xcexcs and 200 xcexcs.
The method according to the invention consists in the fact that the overall capacitance of the recharging capacitors via which the actuator is charged, that is to say in this case the capacitance of at least two recharging capacitors which are connected in parallel and which make possible, for example, a maximum charging time of 200 xcexcs, is reduced at a specific point in time during a charging process by switching off at least one of the parallel recharging capacitors, as a result of which the charging time is shortened.
The following applies to the selection of optimum charging times. The duration of the charging time limits the minimum period of fuel injection. This is critical in particular at high injection pressures because the injected quantity of fuel rises with the fuel pressure that is proportional to the load, given an identical period of injection. In order to achieve a specific injection quantity, in particular a low pre-injection quantity, ever shorter injection periods are therefore necessary as the fuel pressure increases.
On the other hand, in the case of a main injection, the injection quantities are load-dependent and/or pressure-dependent. Given a low load, small injection quantities are required, but given a high load large injection quantities with a high fuel pressure are required. The correlation between the fuel quantity and fuel pressure permits the use of relatively long charging times for the main injection, even in the high load range.
Within certain limits, for example between 100 xcexcs and 200 xcexcs, different charging times of a capacitive actuator have no influence on the injection profile which is relevant for a combustion process, with the exception of delay effects (delays of the start and end of injection) which can be compensated by shifting the timing of the actuation signals.
In accordance with an added mode of the invention, there are the steps of reaching the maximum capacitance of the recharging capacitor using a parallel connection of at least two recharging capacitors; and disconnecting at least one of the two recharging capacitors from the charge source at the specific point in time after the start of the charging process.
In accordance with an additional mode of the invention, there is the step of using the actuator in a fuel injection valve of an internal combustion engine.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for charging a capacitive actuator. The device contains a charge source to be connected to a power source, and a first series circuit disposed between the charge source and the capacitive actuator. The first series circuit has a first charge switch, a first blocking diode connected to the first charge switch, a first recharging capacitor connected to the first blocking diode, and a recharging coil connected to the first recharging capacitor. A reference potential terminal is provided. A discharge switch connects a connecting point of the first blocking diode and the first recharging capacitor to the reference potential terminal. At least one second series circuit is provided and contains a second charge switch, a second blocking diode connected to the second charge switch, and a second recharging capacitor connected to the second blocking diode. The second series circuit is connected in parallel with a third series circuit composed of the first charge switch, the first blocking diode and the first recharging capacitor. A control circuit is connected to and controls the discharge switch, the first charge switch and the second charge switch. A third diode is provided for conducting current in a direction of the discharge switch and is connected between the first and second recharging capacitors. A fourth diode is provided for conducting the current and is disposed between the first recharging capacitor and the discharge switch. The first charge switch and the second charge switch are switched on simultaneously, by the control circuit, to charge the capacitive actuator, and one of the first charge switch and the second charge switch is switched off at a specific point in time for removing the capacitive effect of one of the first and second recharging capacitors.
In accordance with an additional feature of the invention, if the discharge switch is conductive, the capacitive actuator is discharged through the first recharging capacitor and through the second recharging capacitor.
In accordance with a further feature of the invention, the first charge switch, the second charge switch and the discharge switch are MOSFET switches.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for charging a capacitive actuator. The device includes a charge source to be connected to a power source, and a first series circuit disposed between the charge source and the capacitive actuator. The first series circuit has a first charge switch, a first blocking diode connected to the first charging switch and conducts way from the first charge switch, a first recharging capacitor connected to the first blocking diode, and a recharging coil connected to the first recharging capacitor. A reference potential terminal is provided. A second blocking diode is connected to a connection point of the first blocking diode and the first recharging capacitor and conducts current toward the reference potential terminal. A third blocking diode is connected in series with the second blocking diode and has a current conducting direction equivalent to that of the second blocking diode. A discharge switch is connected to the third blocking diode and couples the connecting point of the first blocking diode and of the first recharging capacitor to the reference potential terminal through the second blocking diode and the third blocking diode. A second series circuit is provided and is formed of a second recharging capacitor, a second charge switch connected to the second recharging capacitor, and a fourth blocking diode connected to the second charge switch. The second series circuit is connected between the reference potential terminal and a connecting point of the first recharging capacitor and the recharging coil. The fourth blocking diode conducts current in a direction from the reference potential terminal to the second recharging capacitor. The fourth blocking diode has a cathode connected to the connecting point of the second and third blocking diodes. A control circuit is connected to and controls the discharge switch, the first charge switch and the second charge switch. The first charge switch and the second charge switch are switched on simultaneously, by the control circuit, to charge the capacitive actuator, and one of the first charge switch and the second charge switch is switched off at a specific point in time for removing the capacitive effect of one of the first and second recharging capacitors.
In accordance with another feature of the invention, if the discharge switch is conductive, the capacitive actuator is discharged through the first recharging capacitor, and through the second recharging capacitor and the second charge switch or the fourth blocking diode.
In accordance with a concomitant feature of the invention, the second charge switch is operated inversely with respect to the charge switch, that is to say the second charge switch is switched on when the discharge switch is switched off, and vice versa.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for charging a capacitive actuator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.